The ideal or nominal amount of light or exposure value for exposing a frame of film is specified by the speed, or ISO number, of the film. The camera (or photographer) determines from the ISO number of the film and from the brightness of the scene about to be photographed how much of the scene illumination to admit through the camera shutter onto the film to obtain the ideal exposure. Either the camera's aperture size or shutter exposure time or both must be adjusted to admit just the right amount of light onto the film. The amount of light is increased by increasing either the exposure time or the aperture size or both and is decreased by doing the opposite.
If the camera (or photographer) commits an error in determining the correct shutter exposure time and aperture size used to expose color negative film frame, the negative image on the film is either slightly washed out and of low contrast (in the case of underexposure) or has insufficient contrast between different objects in the image (in the case of an overexposure). The effect is sometimes unavoidable, particularly in cases of high speed photography wherein the shutter exposure time must be extremely short. If the photofinisher is aware of such an error, he can at least partially compensate by changing the amount of light used to expose the film image onto print paper. For example, an underexposed (thin) film negative image should be printed with a less print exposure than normal. The problem is that, absent a skilled photofinishing operator or a sophisticated film image scanner such as those used in the Eastman Kodak Company CLAS 35 film printer, the photofinisher will not necessarily know that a particular film frame has been over- or underexposed.
A more severe problem is that the unusual darkness or lightness of a developed film image is not necessarily an indication of a camera exposure error because the actual scene luminance may have been abnormally dark or light, due to the objects in the scene being unusually light or dark.
In the prior art it has been at least suggested that various scene parameters be recorded on film. Specifically, exposure conditions have been suggested as being recorded optically on the film. The disadvantage here is that such information is not available until after the film is developed. Moreover, the information may not be supplemented, since it must be carefully optically recorded on the film using special exposure devices such as those found in some of the higher quality automatic cameras.
While it has been suggested to record various scene parameters such as shutter speed and aperture size adjacent each frame on the film, it has not been suggested how to make such information meaningful to a photofinisher, since such information only has an absolute meaning if one knows the brightness of the scene and the capabilities of the camera.